As far as methods for preventing the corrosion of iron steels are concerned, galvanizing, which is based on the principle of sacrificial corrosion, is the most effective and economical method. In response to the demand to increase the added values of steel sheets, nearly 90% of thin steel sheets (especially surface-treated steel sheets, with a production of 15 million tons) are accounted for by galvanized steel sheets, and they are used in diverse fields (e.g., building materials, automobiles, household electrical appliances, etc.).
The sacrificial corrosion mechanism of zinc can be expressed as follows: a galvanic cell is formed in a state where two metals (i.e., zinc and iron) are contacted; zinc, which is the baser of the two metals, serves as an anode, whereas iron serves as a cathode; as a result, anodic dissolution of the iron due to the formation of a local cell, which is observed in a case where iron is used alone, is inhibited, and accordingly, the corrosion of the iron or steel is prevented. The corrosion-preventive function is exhausted upon the disappearance of the zinc contacted with the iron, and therefore, it is necessary to inhibit the corrosion of the zinc layer in order to sustain the protective function and effects over an extended period of time. Galvanized sheets are chromate-treated as a mechanism for serving this function.
This chromate treatment corrosion-preventive method, however, has the following problem. When a galvanized steel sheet is chromate-treated, the formation of white rust on the zinc is significantly inhibited, but when the sheet is stored or transported, black rust (also referred to as "darkening") is observed, and the physical appearance of the steel sheet is inferior when it is actually used. This phenomenon also depends on the surface state of the galvanized steel sheet. It has been determined that this tendency is especially noticeable in a case where a skin pass treatment is performed after treating or in the case of a steel sheet plated with a zinc/aluminum alloy containing several percent of aluminum.
As is noted in Japanese Patent Disclosure number Tokkai Sho 59[1984]-177381 (Kokai Hei 3[1991]-49982), a method wherein extremely small quantities of metals are chemically deposited by means of a preliminary treatment in the presence of an aqueous solution containing nickel or cobalt ions prior to a chromate treatment (referred to as the nickel or cobalt "flash treatment") seems promising for preventing the darkening after the chromate treatment.
As a similar technique, a method wherein a galvanized steel sheet is preliminarily treated with an iron ion-containing aqueous solution prior to a chromate treatment has been proposed in Japanese Patent Disclosures Tokkai Sho 62[1987]-20881 and Sho 62[1987]-17183.
The form of the metal thereby deposited may be an elemental metal or its oxide. There are no established theories, however, regarding the mechanism by which this flash treatment prevents the darkening of the galvanized steel sheet as a result of the chromate treatment.
It is mentioned in Proceedings from the 60th Metal Surface Technology Society Lecture Convention, pp. 150-151 that flashed metals are preferentially deposited in zinc crystal grain boundaries, and a chromium-containing compound which has been adhered as a result of a subsequent coat-type chromate treatment is extensively distributed in the grain boundaries, as in the case of the flashed meals. Thus, it may be assumed that the chromium-containing compound is adsorbed on and fixed to the flashed metals due to some interactions between the flashed meals and chromium-containing compound.
It is believed that black zinc rust, which causes darkening, is a basic zinc carbonate represented by (ZnCO.sub.3).sub.x.(Zn(OH.sub.2).sub.y, as in the case of white rust. The difference is thought to be the depletion of oxygen from a stoichiometric point of view. Black rust is a corrosion product obtained in an oxygen-depleted state, and it is presumably formed as the corrosion progresses from the grain boundaries. Thus, it may be expected that the chromium compound which has been concentrated in the grain boundaries by the flashed metals inhibits corrosion from the grain boundaries, thus contributing to the prevention of the production of black rust.